Fan won’t shut off on a C15 cat

Ok so you arent getting power to the solenoid. only possible thing is 2 things imo. You have a break in that harness from that solenoid to the ecm. Or the ecm is an issue. First thing to do is get the wire diagram for your engine. Every truck owner should have a cat manual for their engine on cd. They are like 20 bucks on ebay. SO with the wire diagram, find that lead from the solenoid to the ecm pins and check the voltage at that pin to see if its putting out the voltage you need to fire that soleoid. Yes its very easy to do. But you already said you power power to it and shuts the fan off, so im gonna say that wire is bad in the harness. But worse case scenario, your computer has issues. I recommend buying cat comm adapter and free et download so you can see if the ecm is trying to work. Where are you located

 

Plug it in to Cat E/T , A/C high pressure switch , bad intake manifold temp sensor , etc, can cause the fan to run all the time

 

try unplugging your a/c compressor. or just pull the a/c fuse and see if it stops

 

If you were close to philly pa i was gonna link up with you and scan your ecm with cat et

 

Had a similar issue with a 3406e, was the atmospheric pressure sensor.